The present invention relates to a radial tire for a motorcycle improved in high-speed performance.
In general, widely used for the carcass of a motorcycle tire is a cross-ply structure, in which the carcass plies are crossed with each other so that the carcass cords are laid at 25 to 60 degrees with respect to the circumferential direction of the tire.
The reason is that the requirements for motorcycles are radically different from those for four-wheel vehicles such as passenger cars, particularly, with respect to the operation during cornering.
A motorcycle is tilted during cornering at a extremely large camber angle of max. 40 to 55 degrees with respect to a plane perpendicular to the road surface, while the camber angle in a four-wheel vehicle is generally 3 to 4 degrees.
The centrifugal force at cornering is balanced with a force (camber thrust) generated in parallel to the road surface at the tire ground contacting zone in the direction of the chamber angle when the motorcycle is tilted.
Accordingly, it is necessary for the motorcycle tires to have such characteristics as being able to generate a stable camber thrust to run stably and safely. That is, the motorcycle tires must be laterally stiffened.
In the past, therefore, the cross-ply carcasses were used as described above so as to provide a camber thrust, and radial-ply carcasses having less lateral stiffness were seldom used.
The cross-ply tires have however, such drawbacks that weave phenomenon (transverse vibrations of the motorcycle frame resulting from the lateral stiffness of the tire) occurs during high-speed straight running, and that the wear resistance is low.
As counter-measures to such problems, adjusting the carcass cord arrangement angle and increasing the number of carcass plies have been made, but the drawbacks attributable to the cross-ply structure sill cannot completely be removed as far as the cross-ply structure is employed.
Therefore, recently, to solve the above-mentioned drawbacks of the cross-ply structure, there have been developed radial-ply tires of which the sidewall portions are increased in rigidity to provide lateral stiffness.
However, it has been known that separation of breaker edge portions from the surrounding rubber frequently occurs when a radial-ply structure is applied to a motorcycle tire, unsimilarly to passenger car tires, for the following reasons.
In motor cycle tires, as shown typically in FIG. 2(a), the radius of curvature of the tread is small, that is, the camber value H/L is large, and the breaker comprises at least one ply of cords which ply extends across the whole width of the tread. As a result, the tension in the breaker cords in the tread center regions is different from that in the tread edge regions, and thus stress concentrates in the breaker edges.
Further, because of the high rigidity of the sidewall portions and the small radius of curvature of the tread crown region, the deformation of the tire during running becomes great, particularly from the shoulder portion to the buttress portion as shown in FIGS. 2a and 2b, and the radius of curvature of the inner surface of the tire shoulder is decreased to R2 (FIG. 2b) from R1 (FIG. 2a) when the tread is deformed into the ground contacting state of FIG. 2(b) from the free state of FIG. 2(a).
As the breaker edges B are laid in such greatly deformed regions, a great shear stress is generated around the breaker edges.
If supper high elastic modulus and high tensile strength cords such as steel cords, general aromatic polyamide cords and the like are used as the breaker cords of radial tire, for the purpose of improving durability against such deformation and stress concentration together with the steering stability and the resistance to wear, then the difference in elastic modulus between the breaker cords and the carcass cords becomes more than 3000 kgf/sq.mm, which is too large.
Therefore, the shearing stress induced by the above-mentioned deformations is so increased that the ply separation is more accelerated.
Accordingly, to prevent ply separation by reducing the great step-difference in elastic modulus between the carcass cords and the breaker cords, low elastic modulus breaker cords made of the same material as of the carcass cords or a similar material, for example, nylon cords and the like have sometimes been used.
However, in the tires provided with a nylon breaker, the rigidity of the ground contacting region of the tread becomes low, and the resistance to wear and the steering stability are impaired.
Furthermore, due to the heat generation of the tire during running, the elastic modulus of such low modulus cords becomes still lower. As the result, the growth of the tire is induced, and the durability and the steering stability are deteriorated.
On the contrary, if high elastic modulus cords the same as or similar to the breaker cords, for example, steel cords, general aromatic polyamide cords and the like, are used as the carcass cords, then the rigidity of the tire as a whole become too large so that riding comfort and steering stability become impaired.
It is, therefore, an object of the present invention to provide a radial tire for a motorcycle, in which the above-mentioned problems are completely solved to prevent ply-separation at the breaker edge portions, and to improve the durability of the tire without deteriorating the resistance to wear, the steering stability and the riding comfort.